Power adjusting apparatuses are known, each having a voltage adjusting function of controlling the voltage, in addition to an effective-power adjusting function of controlling the effective power by adjusting the phase angle of the transformer installed at a substation. (Refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-125473.) Also known is an on-load, tap changing apparatus configured to switch the taps of a transformer while the transformer remains excited or loaded, to adjust the voltage value. (Refer to, for example, the JEC-2220 Standard stipulated by Japanese Electrotechnical Committee, JEC). An on-load, tap changing transformers are further known, each configured to switch the taps of the transformer automatically, thereby to make the voltage value of the system, where the transformer is connected, fall within a tolerant range. (Refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-173384.)
Further, substation control systems are known, each having a transformer which has both a voltage adjusting function and an effective-power adjusting function and which is installed between the main bus of the substation and transmission line. In the system, an operator monitors, from time to time, the operating status of the transmission network covering a wide area, and manually moves the voltage tap and phase tap of the transformer, up or down, in accordance with the operating status of the transmission network, thereby suppressing the overload on the transmission lines or optimizing the effective power (flow), while making the voltage value of the power transmission system fall within a tolerant range. (Refer to, for example, the homepage of Tohoku Electric Power Co., Ltd., http://www.tohoku-epco.co.jp/whats/news/1997/71030.htm.)
In any substation control system using a transformer that has a voltage adjusting function and an effective-power adjusting function, the operator fast checks whether the effective power (flow) superimposed in the transmission line, either by switching or due to an accident in the adjacent transmission network, falls within the tolerant range. Then, the operator repeatedly switches the voltage tap and phase tap of the transformer having the voltage adjusting function and effective-power adjusting function, suppressing the effective-power of the power transmission system to a prescribed value, while maintaining the voltage value of the power transmission system within the tolerant range. Hence, the operator needs be highly skilled to make a proper decision in an emergency and to switch the taps by hands immediately.
There is another case that an automatic power regulator and an automatic voltage regulator are used, automatically controlling the voltage and the effective power of the power transmission system in accordance with permissible effective power on the transmission line calculated beforehand, without manual operations by operators.
If an automatic power regulator and an automatic voltage regulator perform automatic control on the voltage and the effective power of the power transmission system, however, the voltage tap and the phase tap will be simultaneously controlled in some cases, without coordination between operations of the automatic power regulator and those of the automatic voltage regulator.
In usual power transmission system, the system voltage rises if the effective power on the transmission line is suppressed. Therefore, if the voltage tap is moved down and the phase tap is simultaneously moved down to reduce the effective power, while the system voltage exceeds the upper limit of a prescribed range, the system voltage may not lower as desired, though the voltage tap is moved down to lower the system voltage. Consequently, the tap control may become unstable and excessive in some cases.
In view of the above, it is desired to provide a substation automatic control system, capable of suppressing the effective power to a prescribed value, while making a automatic power adjusting apparatus and an automatic voltage adjusting apparatus cooperate well with each other, thereby maintaining the system voltage on the transmission line within the tolerant range.